The manual, a reference bible for psychiatrists around the globe, will include pioneering guidelines to help doctors diagnose post-traumatic stress disorder, or PTSD, in children under the age of 6, according to two psychiatrists who worked on the draft.

"When the DSM was written 20 years ago, it was mainly written about adults and field trials were all about adults," said Dr. Charles Zeanah Jr., vice chairman of Child and Adolescent Psychiatry at Tulane University. "The problem was there were children who were very severely traumatized but didn't meet criteria because they weren't included in the studies. Nobody knew what it looked like."

But new research details what PTSD looks like in young children and finds that treatment for these young sufferers can be effective, he said.

The addition of a developmental subtype for any disorder is a first for the manual, which makes the expansion of the PTSD entry so exciting to researchers.

"This is just what I hope will be the beginning of thinking of how different disorders manifest at different points in the life span," he said. "For depression, say, we're not as clear about what the developmental differences ought to be."

Small children develop PTSD at the same rate as adults -- one in four -- and the number of potential sufferers is vast, said Dr. Judith Cohen, a psychiatry professor at Drexel University's College of Medicine.

Consider sexual abuse cases alone: A quarter of all girls and a sixth of all boys have been sexually abused, and about a third of those are preschool-age, she said.

"If we're talking about 25 percent [who] have PTSD, we're talking about a lot of children," Cohen told ABCNews.com. "I think [the DSM change] is going to be a huge contribution to early identification and early prevention of negative outcomes for young children."

And yet because existing DSM criteria doesn't apply to young children, and because of society's tendency to idealize children as resilient, pre-schoolers aren't getting the diagnoses they desperately need, Zeanah said.

Cohen agreed: "The younger the child, the less likely to capture PTSD."

If untreated, exposure to severe trauma in a child or adult can result in devastating health effects, including a permanently altered brain, heart disease and suicide risk that is six times higher than average, said Cohen, who is also the medical director at the Center for Traumatic Stress in Children and Adolescents in the West Penn Allegheny Health System.

Classic PTSD symptoms, in both adults and children, include "re-experiencing" (nightmares, flashbacks), avoidance (trying not to think about the trauma) and hyper-arousal (jumpiness, trouble sleeping), Cohen said.

But young children express symptoms differently.

"Adults might say, 'It's like I'm watching a movie," Zeanah said. "A 2-year-old will repeatedly play out scenes that are aspects of the traumatic experience, often in an agitated and frightened way."

Existing criteria for PTSD aren't appropriate for children, no matter how bright or verbally expressive a young child may be, she said.

"The problem with very young children is they can't adequately describe their internal state," she said. "A child who's 3 or 5, they'll be like, 'Mommy, what does 'ashamed' mean?"

Another common expression of PTSD in adults obviously won't apply to children.

"You're not going to have a 2-year-old talk about a foreshortened future," Zeanah said. "They're very much in the present moment."

Of course, babies and some 2-year-olds can't even talk yet at all. PTSD is typically diagnosable starting at age 1, after traumatic events including invasive hospital procedures, car accidents, dog attacks or witnessing violent events, such as the Newtown massacre, Zeanah said.

Treatment of PTSD in young children varies depending on the age, he said. Methods include a focus on the caregiver to better respond to a very small child's needs, and exposing a slightly older child to trauma reminders until the child's response to it lessens, he said.

For psychiatrists, having standardized diagnostic criteria for PTSD in pre-schoolers means that researchers are talking about the same manifestation of the disorder, whether in Hong Kong or Montana, he said.

It provides a kind of uniformity so researchers can can try to come up with more effective treatments, he said.

"We're so far ahead of even where we were 10 years ago," Cohen said. "Kids do get better, and they go on to lead healthy, happy lives. ... There's reasons for parents to have hope."